Method for connecting plastic pipes and structure for connecting the same

ABSTRACT

Disclosed herein is a method for connecting first (110) and second (120) plastic pipes. The method includes: placing an end of the second plastic pipe (120) on a target portion of the first plastic pipe (110) that is intended to form a connection hole (111), the end of the second plastic pipe (120) being provided with an arch-shaped heating element (130) made of conductive plastic material, electric connection terminals (140) disposed in the heating element (130) on opposite sides of the target portion, and a cover (121) covering at least an outer surface of the arch-shaped heating element (130); supplying power to the electric connection terminals (140) to heat the arch-shaped heating element (130) so that the second plastic pipe (120) is fused to the first plastic pipe (110); and forming the connection hole (111) in the cover (121), the arch-shaped heating element (130) and the side surface of the first plastic pipe (110), so that the first plastic pipe (110) and the second plastic pipe (120) communicate with each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and structure for connectingtwo plastic pipes to each other in a T-shaped form through electricfusion.

2. Description of the Related Art

Electric fusion is generally used to connect pipes to each other, andconnection of pipes will be described below centered on such electricfusion.

In bonding of pipes made of polyethylene, polypropylene or the like tobe used for gas pipes, tap water pipes and other purposes, heat fusionmethods have been mainly used to provide a completely hermetic seal.

Conventional heat fusion methods are classified into a butt fusionmethod, a heat fusion socket method, an electro-fusion socket method, anelectric fusion method, and so on.

Butt fusion is also referred to as butt fusion welding, in which ends ofpipes are butted and fused to each other without use of a separatesocket. In detail, butt fusion is conducted according to the followingsteps of: fixing the pipes in hydraulic or pneumatic clamps in such away that the ends of the pipes to be fusion-bonded face each other;smoothing the ends of the pipes to be fusion-bonded; placing a heatingplate between the ends of the pipes; tightly pressing the ends of thepipes onto the heating plate using hydraulic or pneumatic pressure;heating the heating plate to an appropriate melting point (e.g., about210° C. for polyethylene) to fuse the ends of the pipes; rapidlyremoving the heating plate from the pipes; tightly pressing the fusedends of the pipes onto each other; and cooling the pipes whilemaintaining them in the above state for a predetermined time until thefused ends of the pipes are solidified, thus completing bonding of thepipes.

The above-mentioned method bonds the pipes by melting pipe materials anddoes not require use of a separate socket. However, this method requiresa heat welding machine which is heavy to carry to a working site and hasa complex structure.

Another method uses a typical connection socket, in which the innercircumferential surface of the socket and the outer circumferentialsurfaces of pipes are melted at the same time and then bonded together.That is, the outer circumferential surfaces of the pipes and the innercircumferential surface of the socket are heated at the same time sothat the target surfaces are fused simultaneously, and after they areheated sufficiently, the inner circumferential surface of the socket israpidly fitted over the outer circumferential surfaces of the pipes.Thereafter, the coupled pipe structure is cooled for a predeterminedtime.

Hereinafter, the electro-fusion socket method will be explained. Thisprocess uses a socket which has therein a heating element such as aheating coil. The socket is injection-molded with the heating elementintegrally disposed therein. For example, a copper wire, a copper alloywire, a nichrome wire, etc. which have an appropriate degree ofelectrical resistance is used as the heating coil. Such anelectro-fusion socket integrally provided with an electric resistancewire is manufactured by winding an electric resistance wire coated withthermoplastic resin in a coil form, connecting electric terminals toends of the wire, disposing the wire in a mold, and molding resin. Afterthe socket manufactured in this way is fitted over pipes to be fused,electric current is applied to the socket for a predetermined time by asimple power supply. Then, the outer circumferential surfaces of thepipes and the inner circumferential surface of the socket are fused atthe same time and bonded together.

Such an electro-fusion socket method is also referred to as electricfusion or electro-fusion.

Electric fusion has been generally used for seamed pipes.

At present, electro-fusion seamed pipes are preferred by users becauseof facilitation of work, and various heating elements for theelectro-fusion connector pipes are being developed.

In addition to the foregoing techniques, incorporated hereby byreference in their entirety into this specification, electro-fusiontechniques were also proposed in Korean Patent Laid-open Publication No.10-2002-0012809, entitled “ELECTRONIC FUSION SHEET OF THERMOPLASTIC(E.G., POLYETHYLENE OR POLYPROPYLENE) PIPES USING PRINTED (COATED)HEATING CIRCUIT,” Korean Patent Laid-open Publication No.10-2004-069622, entitled “HEAT GENERATING BUSHING FOR FUSION OF PLASTICPIPES,” Korean Patent Laid-open Publication No. 10-2004-0096757,entitled “FUSION STRUCTURE FOR PLASTIC PIPES,” Korean Patent Laid-openPublication No. 10-2005-0003231, entitled “FUSION STRUCTURE FOR PLASTICPIPES USING FUSION RING” and so on.

Meanwhile, another conventional technique was proposed in PCTInternational Application No. PCT/KR2007/000540, entitled “ELECTRICFUSION PIPING MATERIAL WITH SCREW PRESSING DEVICE”, which was filed bythe applicant of the present invention. This conventional techniqueprovides an electric fusion piping device which includes a pipe jointmade of plastics to be connected with a plastic pipe. The pipe joint hasa thread formed on an inner circumferential surface thereof for threadedcoupling. An annular heating element is provided in an innercircumferential surface of the pipe joint for electric fusion. Theelectric fusion piping device further includes a pressing member. Thepressing member includes a threaded portion which engages with thethread of the pipe joint, and a pressing portion which moves forward inresponse to rotation of the threaded portion to press the annularheating element. Particularly, in this technique, conductive plastic isused as the material of the annular heating element.

Such processes of connecting plastic pipes are mainly conducted in sucha way that linear pipes are arranged in a line and then connected toeach other.

Meanwhile, a plastic pipe may be connected at an end thereof to anotherplastic pipe that has been installed, after a connection hole is formedthe existing plastic pipe. In other words, a plastic pipe may berequired to be connected to the existing plastic pipe in a branch formon a construction site.

In this case, typically, butt fusion method or heat fusion socket methodis mainly used.

PRIOR ART DOCUMENT Patent Document

PCT/KR2007/000540 “ELECTRIC FUSION PIPING MATERIAL MATERIALS WITH SCREWPRESSING DEVICE”

Korean Patent Laid-open Publication No. 10-2002-0012809 “ELECTRONICFUSION SHEET OF THERMOPLASTIC (E.G., POLYETHYLENE OR POLYPROPYLENE)PIPES USING PRINTED (COATED) HEATING CIRCUIT”

Korean Patent Laid-open Publication No. 10-2004-069622 “HEAT GENERATINGBUSHING FOR FUSION OF PLASTIC PIPES”

Korean Patent Laid-open Publication No. 10-2004-0096757 “FUSIONSTRUCTURE FOR PLASTIC PIPES”

Korean Patent Laid-open Publication No. 10-2005-0003231 “FUSIONSTRUCTURE FOR PLASTIC PIPES USING FUSION RING”

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a method and structure for connecting twoplastic pipes to each other in a T-shaped form through electric fusionand, particularly, a method and structure for connecting a plastic pipe,in a branch shape, to another plastic pipe that has been alreadyinstalled.

In order to accomplish the above object, in an aspect, the presentinvention provides a method for connecting first and second plasticpipes to each other in such a way that an end of the second plastic pipeis connected to a side surface of the first plastic pipe to communicatethe first plastic pipe with the second plastic pipe, the methodincluding: placing the end of the second plastic pipe on a portion of anouter surface of the first plastic pipe, the portion being intended toform a connection hole therein, the end of the second plastic pipe beingprovided with: an arch-shaped heating element made of conductive plasticmaterial that generates heat using electricity applied thereto, thearch-shaped heating element having an arch cross-sectional shape tocover the connection hole; a pair of electric connection terminalsdisposed parallel to each other in the arch-shaped heating element onopposite sides of the portion intended to form the connection holetherein; and a cover covering at least an outer surface of thearch-shaped heating element; supplying power to the electric connectionterminals to heat the arch-shaped heating element so that the secondplastic pipe is fused to the first plastic pipe; and forming theconnection hole in the cover, the arch-shaped heating element and theside surface of the first plastic pipe after supplying the power isconducted, so that the first plastic pipe and the second plastic pipecommunicate with each other through the connection hole.

The electric connection terminals may be in line contact with thearch-shaped heating element, and shortest distances between the electricconnection terminals at all points of any one of the electric connectionterminals may be equal to each other.

In another aspect, the present invention provides a structure forconnecting first and second plastic pipes to each other in such a waythat an end of the second plastic pipe is fused to a side surface of thefirst plastic pipe to communicate the first plastic pipe with the secondplastic pipe through a connection hole, the structure including: anarch-shaped heating element made of conductive plastic material thatgenerates heat using electricity applied thereto, the arch-shapedheating element closing the end of the second plastic pipe and having anarch cross-sectional shape to cover the side surface of the firstplastic pipe; a pair of electric connection terminals disposed parallelto each other in the arch-shaped heating element on opposite sides of aportion intended to form the connection hole therein; and the secondplastic pipe provided on the end thereof with a cover covering at leastan outer surface of the arch-shaped heating element.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a perspective view showing a first plastic pipe that is justbefore being placed on a second plastic pipe according to a comparativeexample of the present invention;

FIG. 2 is a perspective view showing the first plastic pipe placed onthe second plastic pipe of FIG. 1;

FIG. 3 is a longitudinal sectional view of FIG. 2;

FIG. 4 is a sectional view taken along line A-A of FIG. 3;

FIG. 5 is a perspective view of an arch-shaped heating element used inthe comparative example of the present invention;

FIG. 6 is a conceptual view showing conditions in which power is appliedto the arch-shaped heating element of FIG. 5;

FIG. 7 is a perspective view showing a first plastic pipe that is justbefore being placed on a second plastic pipe according to an embodimentof the present invention;

FIG. 8 is a perspective view showing the first plastic pipe placed onthe second plastic pipe of FIG. 7;

FIG. 9 is a longitudinal sectional view of FIG. 8;

FIG. 10 is a sectional view taken along line B-B of FIG. 9;

FIG. 11 is a perspective view of an arch-shaped heating element used inthe embodiment of the present invention;

FIG. 12 is a conceptual view showing conditions in which power isapplied to the arch-shaped heating element of FIG. 11;

FIG. 13 is a view corresponding to FIG. 9, but showing a connection holeformed after power is supplied to the arch-shaped heating element;

FIG. 14 is a sectional view taken along line B-B of FIG. 13; and

FIG. 15 is a view corresponding to FIG. 14, but showing a cover havingno inner cover part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will bedescribed in detail with reference to the attached drawings, such thatthose skilled in the art can easily implement the present invention. Thepresent invention is realized in various manners and is not limited tothe following embodiment. Furthermore, in the drawings, portions whichare not related to the present invention should be omitted to explainthe present invention more clearly. Reference should be made to thedrawings, in which similar reference numerals are used throughout thedifferent drawings to designate similar components.

In the specification, when the explanatory phrase “a part includes acomponent” is used, this means that the part may further include othercomponents rather than excluding the components unless specialexplanation is given.

A comparative example introduced by the inventor of the presentinvention will be explained, and problems thereof will be described.

FIG. 1 is a perspective view showing a first plastic pipe that is justbefore being placed on a second plastic pipe according to a comparativeexample of the present invention;

FIG. 2 is a perspective view showing the first plastic pipe placed onthe second plastic pipe of FIG. 1. FIG. 3 is a longitudinal sectionalview of FIG. 2. FIG. 4 is a sectional view taken along line A-A of FIG.3. FIG. 5 is a perspective view of an arch-shaped heating element usedin the comparative example of the present invention. FIG. 6 is aconceptual view showing conditions in which power is applied to thearch-shaped heating element of FIG. 5.

The first plastic pipe 110 has a tubular shape and is open on oppositeends thereof. A connection hole 111 is formed in a medial portion of thefirst plastic pipe 110. Of course, another plastic pipe, a seamed pipeor other kind of piping substance can be connected to each end of thefirst plastic pipe 110. The first plastic pipe 110 is in a state ofbeing already installed on a site. The second plastic pipe 120 is to beconnected to the first plastic pipe 110.

The connection hole 111 is used to communicate the first plastic pipe110 with the second plastic pipe 120.

A separate structure for connection to another plastic pipe may beprovided on each of opposite ends of the first plastic pipe 110, butthis is irrelevant to the core of the present invention, so furtherexplanation thereof will be omitted.

The second plastic pipe 120 is connected to the first plastic pipe 110.

The second plastic pipe 120 also has a tubular structure which is openon opposite ends thereof.

A first end of the opposite ends of the second plastic pipe 120 isplaced on the first plastic pipe 110 such that the second plastic pipe120 communicates with the connection hole 111 of the first plastic pipe110.

Although the second plastic pipe 120 is illustrated as having a typicalpipe shape, it may have a seamed pipe shape.

To achieve the above-mentioned placement of the second plastic pipe 120,as shown in this comparative example, the second plastic pipe 120 may beoriented perpendicular to the first plastic pipe 110. Alternatively, thesecond plastic pipe 120 may be inclined to the first plastic pipe 110.

Meanwhile, a cover 121 is provided on the first end of the secondplastic pipe 120. The cover 121 has a shape corresponding to the outercircumferential surface of the first plastic pipe 110. The cover 121 isplaced on the first plastic pipe 110 in such a way that the cover 121partially covers the outer circumferential surface of the first plasticpipe 110.

Furthermore, the cover 121 has a shape in which it radially extends fromthe first open end of the second plastic pipe 120.

To correspond to the shape of the outer circumferential surface of thefirst plastic pipe 110, as shown in FIG. 4, the cross-section of thecover 121 must have an arch shape because the first plastic pipe 110 iscylindrical.

Of course, the central portion of the cover 121 is open.

An arch-shaped heating element 130 is provided in the cover 121 of thesecond plastic pipe 120.

The arch-shaped heating element 130 must have a shape corresponding tothe outer circumferential surface of the first plastic pipe 110 and, asshown in FIG. 3 or 5, has an arch-shaped cross-section.

Also, the arch-shaped heating element 130 is disposed in such a way thatit encloses the connection hole 111 of the first plastic pipe 110. Asshown in FIG. 4, corresponding to the connection hole 111 of the firstplastic pipe 110, an opening 131 is formed in a central portion of thearch-shaped heating element 130.

Furthermore, the arch-shaped heating element 130 is made of conductiveplastic material which generates heat when electricity is suppliedthereto. Such conductive plastic material has been stated in thedescription of the conventional art, and further explanation thereof isdeemed unnecessary.

The cover 121 of the second plastic pipe 120 encloses both inner andouter surfaces of the arch-shaped heating element 130.

The cover 121 of the second plastic pipe 120 includes an outer coverpart 121 a which is provided on the outer surface of the arch-shapedheating element 130, and an inner cover part 121 b which is provided onthe inner surface of the arch-shaped heating element 130.

When the arch-shaped heating element 130 generates heat, the inner coverpart 121 b is melted and thus integrated with the outer surface of thefirst plastic pipe 110.

To supply electricity to the arch-shaped heating element 130, a pair ofelectric connection terminals 140 are provided in the arch-shapedheating element 130.

In this comparative example, facing each other, the electric connectionterminals 140 are disposed on opposite sides of the opening 131 andoriented parallel to each other.

By virtue of the above-mentioned disposition of the electric connectionterminals 140, the shortest distance between the electric connectionterminals 140 disposed in the arch-shaped heating element 130 can beeasily maintained constant at all points of the electric connectionterminals 140.

To connect the electric connection terminals 140 linearly disposed inthe arch-shaped heating element 130 to an external power supply, anauxiliary connection terminal 141 extends from each electric connectionterminal 140 to the outside of the arch-shaped heating element 130. Theshape and location of the auxiliary connection terminal 141 can bechanged in various ways depending on embodiments.

After the elements have been disposed as described above, the electricconnection terminals 400 are connected to the power supply (not shown)and power is applied to the electric connection terminals 400 for apredetermined time. Then, the arch-shaped heating element 130 is heatedby current that flows along the electric connection terminals 400. Thearch-shaped heating element 130 and the cover 121, particularly, theinner cover part 121 b, are fused to the outer circumferential surfaceof the first plastic pipe 110 by heat generated from the arch-shapedheating element 130 and are thus integrated therewith.

FIG. 6 is a conceptual plan view showing the structure of the electricconnection terminals 140 and the arch-shaped heating element 130 havingthe opening 131.

As shown in the drawing, in the case of portions other than the opening131, current can linearly flow along the shortest distance between theelectric connection terminals 140.

However, in the case of a portion defining the opening 131, currentflows along the shortest distance around the opening 131 between theelectric connection terminals 140.

Therefore, much more current flows in portion A, while little currentflows in portion B.

Meanwhile, in the arch-shaped heating element 130 that is made ofconductive plastic material, at an initial stage of a heating process,as the temperature of the heating element increases, the matrix-phasepolymer of the heating element 130 expands. Thereby as the distancesbetween conductive particles connected to the polymer are increased,resistance is increased, so that the heating element 130 has PTC(positive temperature coefficient) characteristics.

Furthermore, as the temperature of the heating element 130 increases,while the temperature passes through the melting point at which thematrix-phase polymer of the heating element 130 begins to be melted, theresistance becomes maximal at the melting point and thereafter israpidly reduced. The reason for this is because of the fact that when anelectric field is applied to polymer in a liquid state, conductiveparticles are connected to each other again in the direction of theelectric field. In this regard, the arch-shaped heating element has NTC(negative temperature coefficient) effect at a temperature over apredetermined level.

Therefore, in the arch-shaped heating element 130, even when constantvoltage is applied to the electric connection terminals 140, portion Ais more rapidly heated and melted than the other portions. Therefore,when the other portions are heated and melted, portion A is already inthe molten state. In this state, portion A receives power (that is,heat), so that the temperature thereof continuously increases and,ultimately, exceeds the fire point of the matrix-phase polymer. As aresult, the matrix-phase polymer is decomposed and thus easily broken,whereby the shape thereof cannot be maintained.

Furthermore, even when constant voltage is applied to the electricconnection terminals 140 of the arch-shaped heating element 130, littlepower is applied to the portion B, so that portion B is not reliablyfused.

As such, the comparative example is problematic in that both portion Aand portion B are not reliably fused.

Hereinafter, an embodiment of the present invention will be described indetail.

FIG. 7 is a perspective view showing a first plastic pipe 110 that isjust before being placed on a second plastic pipe 120 according to anembodiment of the present invention. FIG. 8 is a perspective viewshowing the first plastic pipe placed on the second plastic pipe of FIG.7. FIG. 9 is a longitudinal sectional view of FIG. 8. FIG. 10 is asectional view taken along line B-B of FIG. 9. FIG. 11 is a perspectiveview of an arch-shaped heating element used in the embodiment of thepresent invention. FIG. 12 is a conceptual view showing conditions inwhich power is applied to the arch-shaped heating element of FIG. 11.FIG. 13 is a view corresponding to FIG. 9, but showing a connection holeformed after power is supplied to the arch-shaped heating element. FIG.14 is a sectional view taken along line B-B of FIG. 13. FIG. 15 is aview corresponding to FIG. 14, but showing a cover having no inner coverpart.

A method of connecting plastic pipes according to the embodiment of thepresent invent includes three steps, in detail, (1) plastic pipe placingstep, (2) power supply step and (3) connection hole forming step.

(1) Plastic Pipe Placing Step

An arch-shaped heating element 130, a pair of electric connectionterminals 140 and the second plastic pipe 120 are placed on the firstplate pipe 110 that has been already installed, without a connectionhole 111 being formed in the first plate pipe 110.

In this step, the connection hole 111 is not formed in the first plasticpipe 110.

The arch-shaped heating element 130 is placed in such a way that itcovers a portion of the first plate pipe 110 in which the connectionhole 111 is intended to be formed. The arch-shaped heating element 130has a curved plate shape without having an opening 131.

The electric connection terminals 140 are oriented parallel to eachother and disposed on opposite sides of the portion of the first platepipe 110 in which the connection hole 111 is intended to be formed.

In this embodiment, the cover 121 of the second plastic pipe 120encloses the arch-shaped heating element 130, in other words, coversboth inner and outer surfaces of the arch-shaped heating element 130. Asneeded, the cover 121 may be configured such that it covers only theouter surface of the arch-shaped heating element 130.

The second plastic pipe 120 of this embodiment is configured in such away that an end thereof is closed by the cover 121. Further, the end ofthe second plastic pipe 120 is also covered with the arch-shaped heatingelement 130.

Meanwhile, in this embodiment, although the arch-shaped heating element130 and the electric connection terminals 140 have been illustrated asbeing integrally formed along with the cover 121 of the second plasticpipe 120, the second plastic pipe 120 and the arch-shaped heatingelement 130 may be manufactured through separate processes andindividually installed on a site.

(2) Power Supply Step

After the plastic pipe placing step, the electric connection terminals400 are connected to the power supply (not shown) and power is appliedto the electric connection terminals 400 for a predetermined time. Then,the arch-shaped heating element 130 is heated by current that flowsalong the electric connection terminals 400. The arch-shaped heatingelement 130 and the cover 121, particularly, the inner cover part 121 b,are fused to the outer circumferential surface of the first plastic pipe110 by heat generated from the arch-shaped heating element 130 and arethus integrated therewith.

Here, as shown in FIGS. 11 and 12, because the arch-shaped heatingelement 130 has a plate shape having no opening 131, the entire portionbetween the electric connection terminals 400 is uniformly heated,melted and fused.

(3) Connection Hole Forming Step

After the plastic pipe placing step, as shown in FIGS. 13 and 14, aconnection hole 111 is formed at a predetermined position in thearch-shaped heating element 130, the cover 121 and the first plasticpipe 110 such that the first plastic pipe 110 communicates with thesecond plastic pipe 120.

Thereby, the first plastic pipe 110 and the second plastic pipe 120 areintegrated with each other by fusion in such a way that they communicatewith each other.

According to the above-mentioned method, the second plastic pipe 120 canbe connected to the first plate pipe 110 with satisfactory fusionquality.

Meanwhile, as necessary, the cover 121 may include only the outer coverpart 121 a without having the inner cover part 121 b. In this case, asshown in FIG. 15 that is a view corresponding to FIG. 14, thearch-shaped heating element 130 is directly fused to and integrated withthe outer circumferential surface of the first plastic pipe 110.

As described above, the present invention provides a method ofconnecting two plastic pipes in a T-shaped form, particularly, a methodof connecting a second plastic pipe, in a branch shape, to a first platepipe that has been already installed. According to the plastic pipeconnection method, the fusion quality can be markedly enhanced.Furthermore, the present invention provides a connection structure forrealizing the plastic pipe connection method.

Although the preferred embodiment of the present invention has beendisclosed, those skilled in the art will appreciate that variousmodifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims. Therefore, it should be understood that thepreferred embodiment is only for illustrative purposes and does notlimit the bounds of the present invention. For instance, componentswhich have been illustrated as being integrated with each other may beimplemented in a separate structure, and components which have beenillustrated as being separately provided may be provided in anintegrated structure.

Therefore, it is intended that the bounds of the present invention aredefined by the accompanying claims, and various modifications, additionsand substitutions, which can be derived from the meaning, scope andequivalent concepts of the accompanying claims, fall within the boundsof the present invention.

What is claimed is:
 1. A method for connecting first and second plasticpipes to each other in such a way that an end of the second plastic pipeis connected to an arcuate side surface of the first plastic pipe tocommunicate the first plastic pipe with the second plastic pipe, the endof the second plastic pipe having a connection hole while the firstplastic pipe does not initially have a connection hole, the methodcomprising: providing an arch-shaped heating element made of conductiveplastic material that conforms in shape to the arcuate side surface ofthe first plastic pipe, the heating element not initially having a holetherein, the heating element generating heat when electricity is appliedthereto; disposing a pair of electrical connection terminals parallel toeach other in opposite sides of the heating element; covering at leastan outer surface of the heating element with a cover; connecting thecover and heating element on the end of the second plastic pipe; placingthe second plastic pipe, the cover and the heating element on thearcuate surface of the first plastic pipe in the area where a connectionbetween the first and second plastic pipes is desired; supplying powerto the electric connection terminals to heat the arch-shaped heatingelement so that the second plastic pipe is fused to the first plasticpipe; and thereafter, forming a connection hole in the cover, thearch-shaped heating element and the side surface of the first plasticpipe after supplying the power is conducted, so that the first plasticpipe and the second plastic pipe communicate with each other through theconnection hole.
 2. The method as set forth in claim 1, wherein theelectric connection terminals are in line contact with the arch-shapedheating element, and shortest distances between the electric connectionterminals at all points of any one of the electric connection terminalsare equal to each other.
 3. A structure for connecting first and secondplastic pipes to each other in such a way that an end of the secondplastic pipe is fused to a side surface of the first plastic pipe tocommunicate the first plastic pipe with the second plastic pipe througha connection hole, the structure comprising: an arch-shaped heatingelement made of conductive plastic material that generates heat usingelectricity applied thereto, the arch-shaped heating element having anarch cross-sectional shape to conform with the side surface of the firstplastic pipe; a pair of electric connection terminals disposed parallelto each other in the arch-shaped heating element on opposite sides of aportion of the heating element intended to form the connection holetherein; a cover covering at least an outer surface of the arch-shapedheating element; and the cover and heating element being connect to anend of the second pipe and closing the hold in the second pipe, whereina connection hole in the cover, the arch-shaped heating element and theside surface of the first plastic pipe can be formed, after supplyingthe power to the electric connection terminals, so that the firstplastic pipe and the second plastic pipe communicate with each otherthrough the connection hole.